Recently, an ECR (electron cyclotron resonance) plasma processing method, which causes a microwave discharge by use of resonance between a cyclotron motion of electrons and a microwave in a magnetic field, has attracted attention as one of the plasma-based film forming and etching methods. According to this method, a high density plasma can be generated in a high vacuum by an electrodeless discharge. Thus, this method is advantageous in that high speed surface treatment can be performed, and in that contamination of a semiconductor wafer (hereinafter referred to as xe2x80x9cwaferxe2x80x9d) or the like is unlikely.
This invention relates to a plasma processing apparatus and a plasma processing method, more specifically, to a plasma processing apparatus and a plasma processing method in which electron cyclotron resonance is utilized.
An example of a conventional plasma processing apparatus for performing the ECR plasma treatment will be described with reference to FIG. 5A which illustrates film formation. In this plasma processing apparatus, microwaves of, for example, 2.45 GHz supplied through a waveguide 13 are introduced into a plasma generation chamber 1A of a vacuum vessel 1 via a microwave transmitting window 14. Simultaneously, a magnetic field of a predetermined magnitude, e.g., 875 gausses, is applied by an electromagnetic coil 10 so that a plasma-generating gas, such as an Ar gas or an O2 gas, is converted into a high density plasma by the interaction between the microwaves and the magnetic field (i.e., electromagnetic cyclotron resonance). The resulting plasma activates a reactive processing gas, e.g., SiH4 gas, introduced sideways into a film forming chamber 1B of the vacuum vessel 1 to form active species. The active species carry out both of sputter etching of and deposition on the surface of a silicon wafer W laid on a bearing stand 11 connected to a high frequency power source unit 12. The contradictory sputter etching operation and deposition operation are controlled such that the deposition operation is predominant from a macroscopic viewpoint. As a whole, deposition takes place.
Generally, the execution of uniform treatment requires a uniform plasma distribution on the wafer surface. For this purpose, a plasma density distribution D1 having mountain-shaped ends needs to be obtained near the ECR point above the wafer. The inventor of the present invention is aware of this fact. Formation of such a plasma density distribution results in a group of magnetic lines of force spreading toward the wafer. The degree of their spread is greater at the peripheral edge than at the center of the wafer. Thus, the plasma density distribution becomes flat on the wafer.
However, the inner water surface of the conventional plasma generation chamber 1A is made of a metal. Thus, the microwaves introduced into the plasma generation chamber 1A are reflected by its inner wall surface, whereupon standing waves as shown by arrows in FIG. 5A appear. As a result, the electric field strength distribution in the plasma generation chamber 1A is overlaps of the strength distribution of the microwaves introduced from the outside, and the strength distribution in cavity modes by the plasma generation chamber 1A that plays the role of a cavity. This poses the problem that, as shown in FIG. 5B, the plasma density D1 at the mountain-shaped ends becomes nonuniform relative to the circumferential direction, making uniform film formation impossible.
The present invention has been accomplished to solve the above problem, and the object of the present invention is to provide a plasma processing apparatus and a plasma processing method, each of which enables reduction of reflected waves in a vacuum chamber to suppress standing waves, thereby easily controlling the plasma density so that uniform treatment can be performed.
To attain this object, the present invention provides with a plasma processing apparatus of a type that microwaves are introduced from a waveguide into a vacuum chamber via a transmissive window, a processing gas is converted into a plasma by means of the microwaves, and that a object to be processed is processed with the plasma. The apparatus is characterized in that an electromagnetic wave absorber is provided on an inner wall surface of the vacuum chamber.
Preferably, the electromagnetic wave absorber is composed of a resistor, a dielectric having a large dielectric loss, or a magnetic material having a large magnetic loss, or a combination of these.
According to another aspect of the present invention, there is provided a plasma processing method including the steps of: introducing microwaves from a waveguide into a vacuum chamber via a transmissive window; absorbing the microwaves to an electromagnetic wave absorber in the vacuum chamber to suppress reflected waves, thereby generating a plasma in an area opposed to a object to be processed, the area being located between the transmitting window and the object, the plasma having a nonuniform plasma density distribution with a plasma density higher at a peripheral area of the plasma than at a central area of the plasma; and treating the object with the plasma.
According to the present invention, the microwaves introduced into the vacuum chamber via the transmitting window are absorbed to the electromagnetic wave absorber. Thus, reflected waves in the vacuum chamber are suppressed, and control of a plasma density distribution is facilitated. Hence, uniform treatment can be performed.